1. Field of the Invention
The present invention relates generally to a device and method for performing handoff in a mobile communication system, and more particularly, to a novel device and method for implementing handoff when a mobile station travels from a cell of the async mobile communication system to a cell of the sync mobile communication system.
2. Description of the Related Art
The async mobile communication system is a system including a terminal operating in accordance with the IMT-2000 specification being standardized in 3GPP (third generation partnership project). The sync mobile communication system is a system operating in accordance with the IMT-2000 specification being standardized in 3GPP2, and includes IS-95 and J-STD008, the next generation sync systems. These two systems are becoming increasingly harmonized and there is thus a need for various technologies that are compatible with both systems. One of such technologies is related to handoff that may happen between the sync mobile communication system and the async mobile communication system.
Handoff is a technology that enables users to continuously receive call service without interruption when a mobile station travels from a present cell to an adjacent cell during the call service in a mobile communication system. Handoffs are classified as soft hand off and hard hand off. In the soft hand off, the mobile station maintains a call using both a channel assigned by a target base station and a channel assigned by the present base station in service. Eventually, the mobile station abandons one of the two channels, whose quality has a value lower than a threshold. In the hard hand off, a channel assigned by the present base station in service is first released, and then connection to an adjacent base station is attempted.
Until now, development of the handoff has been concentrated on the sync mobile communication system. But, with the emergence of the async mobile communication system, research has been undertaken regarding handoff between sync and async mobile communication systems.
The mobile station usually acquires information about adjacent cells and informs the base station of the information when a pilot signal received from one of the adjacent cells has a higher strength than a threshold or a handoff direction message is received from the base station. The information sent to the base station is used as information about the handoff performed when the mobile station travels from a present cell to the adjacent cell during a call service. A hard handoff takes place normally when the mobile station travels from a cell of the async mobile communication system to a cell of the sync mobile communication system. For the hard handoff, the mobile station interrupts a call service from the async mobile communication system while it acquires information about the adjacent cells.
Conventionally, the mobile station has to perform the following procedure in order to interpret information about the sync mobile communication system. First, the mobile station interprets a sync signal message stored in the sync signal frame transferred from the sync channel of the sync mobile communication system. The transmission bit per 80 ms frame of the sync signal frame is 96 bits, and the sync signal message including information the mobile station can communicate with the sync mobile communication system, has a length of 221 bits. Thus the mobile station needs at least 240 ms (80 ms×3) for interpreting the message. The above-mentioned specifications are included in the TIA/EIA-IS-2000.5 standard that define the sync mobile communication system.
Hereinafter, a base station of the sync mobile communication system will be referred to as “sync base station” and a base station of the async mobile communication system will be called “async base station”.
FIG. 2 illustrates a procedure for the mobile station in communication with a present async base station to acquire information about the adjacent sync base stations. Referring to FIG. 2, the mobile station receives from an async base station a direction message to detect information about sync base stations adjacent to the async base station, in step 201. Then, the mobile station sets to detect information about the adjacent sync base stations in step 203, and detects pilot signals from the adjacent sync base stations in step 205. The mobile station determines in step 207 whether a pilot signal having a highest peak value is detected. Upon failure to detect such a pilot signal of highest peak value, the mobile station returns to step 205. If a highest peak value of a pilot signal is detected, the mobile station proceeds to step 209 to receive sync frames through the forward sync channel of the sync base station from which the pilot signal having the highest peak value has been detected. In this case, the mobile station has to receive at least three sync frames from the sync base station in order to receive all sync signal messages. For example, the mobile station takes at least 240 ms in receiving the sync frames from the sync base station having a channel structure shown in FIG. 1 and, during the frame reception time, interrupts the communication with the async base station. Taking a long time in performing the procedure of FIG. 2 may therefore result in a detrimental effect such as a loss of data communicated between the async base station and the mobile station. Moreover, the mobile station does not necessarily receive messages at the time when it begins to receive the sync frames. Including the time of waiting for the time in the unit of 240 ms, the mobile station has to interrupt communication with the async base station for about 513.3 ms on the assumption that there is no error in the received frames. Therefore, a loss of data communicated between the sync base station and the mobile station is inevitable if the mobile station takes a long time in performing the procedure of FIG. 2. This does not meet the compressed mode defined in the async system in order to monitor other frequency bands suggested in the async standards.
FIG. 1 illustrates an exemplary construction of the respective channels communicated between a mobile station and a base station, and a channel communication device for the respective channels in a code division multiple access (CDMA) communication system, which is one of the sync mobile communication systems according to the prior art. The respective channels shown in FIG. 1 are illustrated focusing on a transmitter.
To describe the channel construction of a base station, a controller 101 enables/disables an operation of the individual channel generators, processes a message communicated between a physical layer and the base station, and communicates messages with the upper layer. Pilot channel generator 103, sync channel generator 104, and paging channel generator 107 are devices for generating common channel information shared among the users in a single cell or a plurality of cells. Dedicated control channel (DCCH) generator 102, fundamental channel (FCH) generator 108 and supplemental channel (SCH) generator 109 are devices for generating dedicated channel information assigned differently to the users.
The dedicated control channel generator 102 processes various control messages received on a forward dedicated control channel DCCH and sends them to a mobile station. The messages on the forward dedicated control channel include radio link protocol (RLP) frames or various control messages used in the IS-95B standard, and medium access control messages (MAC) related to a packet data service control, i.e., assigning or releasing supplemental channels. Power control signals can be transmitted on the dedicated control channel instead of the fundamental channel, in which case the power control signals are included in the control messages. On the forward dedicated control channel, the dedicated control channel generator 102 negotiates with the base station in regard to a data rate to be used for a supplemental channel or, if orthogonal codes are used for the supplemental channel, gives a direction to change the orthogonal codes. The forward dedicated control channel is spread with one of the unused orthogonal codes among those not assigned to the pilot channel generator 103, sync channel generator 104, or paging channel generator 107. The RLP frame provides a service for successful transmission of an octet stream. The RLP may be classified into transparent RLP and non-transparent RLP. The transparent RLP does not retransmit an erroneously transmitted frame but informs the upper layer of the time and position of the erroneously transmitted frame. The non-transparent RLP involves error correction.
The pilot channel generator 103 processes information received on a forward pilot channel and sends the received information to the mobile station. The forward pilot channel always transmits logic signals of all 0's or 1's. It is assumed herein that the pilot channel transmits logic signals of all 0's. The pilot channel signal enables the mobile station to rapidly acquire initial synchronization for new multiple paths and estimate channels. The pilot channel is spread with one specific orthogonal code previously assigned thereto.
The sync channel generator 104 processes information received on a forward sync channel and sends the received information to the mobile station. Information on the sync channel enables every mobile station in a cell to acquire initial time and frame synchronizations. The forward sync channel is spread with one specific Walsh code previously assigned thereto.
The paging channel generator 107 processes information received on a forward paging channel and sends the received information to the mobile station. Information on the paging channel is all information necessary prior to establishment of traffic channels. The forward paging channel is spread with one of orthogonal codes previously assigned thereto.
The fundamental channel generator 108 processes information received on a forward fundamental channel and sends the received information to the mobile station. Information on the forward fundamental channel may include a variety of control messages (L3 signaling) used in the IS-95B standard and power control signals, other than the voice signal. If necessary, such information may include RLP frames and MAC messages. The fundamental channel has a data rate of 9.6 kbps or 14.4 kbps and, according to circumstances, has a variable data rate such as 4.8 kbps or 7.2 kbps as ½ of the given data rate; 2.4 kbps or 3.6 kbps as ¼ of the data rate; or 1.2 kbps or 1.8 kbps as ⅛ of the data rate. Such a variable data rate must be detected by the receiving unit. The forward fundamental channel is spread with orthogonal codes not assigned to the pilot channel generator 103, sync channel generator 104, or paging channel generator 107.
The supplemental channel generator 109 processes information received on a forward supplemental channel and sends the received information to the mobile station. Information on the forward supplemental channel includes RLP frames, packet data and the like. The supplemental channel generator 109 has a data rate of more than 9.6 kbps. The supplemental channel generator 109 has a scheduled data rate, i.e., the base station communicates with the mobile station at a data rate determined under negotiation with the mobile station through the dedicated control channel. The forward supplemental channel is spread with orthogonal codes not assigned to the pilot channel generator 103, sync channel generator 104, or paging channel generator 107. The fundamental channel and the supplemental channel become traffic channels.
An adder 110 adds in-phase channel transmission signals on the forward link from dedicated control channel generator 102, fundamental channel generator 108 and supplemental channel generator 109 to transmission signals from pilot channel generator 103, sync channel generator 104 and paging channel generator 107. An adder 111 adds together quadrature-phase channel transmission signals output from dedicated control channel generator 102, fundamental channel generator 108 and supplemental channel generator 109. A spreading modulator 112 multiplies the transmission signals from the adders 110 and 111 by a spreading sequence and ascent frequency converts the transmission signals. A receiver 123 frequency converts the respective channel signals of the mobile station on the reverse link with a base band and then despreads the signals through multiplication of the converted signals by a spreading sequence. The construction of the reverse link channel receivers provided in the base station are omitted in FIG. 1.
Now to describe the channel construction of the mobile station, a controller 114 enables/disables the operation of the individual channel generators, processes a message communicated by the mobile station, and communicates messages with the upper layer.
A dedicated control channel generator 115 processes various control messages received on a reverse dedicated control channel and sends them to a base station. The messages on the reverse dedicated control channel include radio link protocol (RLP) frames or various control messages used in the IS-95B standard, and medium access control messages (MAC) related to a packet data service control, i.e., assigning or releasing supplemental channels. For a reverse link, power control signals are not separately transmitted on the dedicated control channel because they are inserted in a pilot channel for transmission. On the reverse dedicated control channel, the dedicated control channel generator 115 negotiates with the base station in regard to a data rate to be used for a supplemental channel. The reverse dedicated control channel generator 115 spreads the individual channels with unique orthogonal codes previously assigned thereto to discriminate the channels and spreads the signals from the users with unique PN codes to discriminate the users. Thus different orthogonal codes are assigned to a dedicated control channel, a pilot channel, an access channel, a fundamental channel and a supplemental channel in order to discriminate the respective channels, and the respective orthogonal codes used for every channel are shared among the users. For example, an orthogonal code used for the dedicated control channel is shared among all users to discriminate the dedicated control channel.
The reverse dedicated control channel has a fixed data rate of 9.6 kbps, which prevents any performance deterioration in determining the data rate and eliminates a data rate determination circuit, reducing complexity of the receiver. Also, the reverse dedicated control channel has the same data rate as the basic data rate of voice signals, i.e., 9.6 kbps, thus maintaining the same service diameter as the basic voice service.
A pilot channel generator 116 processes information received on a reverse pilot channel and sends the received information to the base station. Like the forward pilot channel, the reverse pilot channel enables rapid acquisition of initial synchronization for new multiple paths and channel estimation. The reverse pilot channel also transmits reverse power control information by adding power control signals to the pilot signal at a defined time.
An access channel generator 117 processes information received on a reverse access channel and sends the received information to the base station. The information on the access channel includes control messages and all information about the mobile station required by the base station prior to establishment of a traffic channel.
A fundamental channel generator 118 processes information received on a reverse fundamental channel and sends the received information to the base station. Information on the reverse fundamental channel normally includes voice signals. Such information may include a variety of control messages (L3 signaling) used in the IS-95B standard as well as voice signals. If necessary, the information may include RLP frames and MAC messages. For a reverse link, power control signals are not separately transmitted on the access channel because they are inserted in the pilot channel for transmission. The fundamental channel has a fixed data rate of 9.6 kbps or 14.4 kbps and, according to circumstances, has a variable data rate such as 4.8 kbps or 7.2 kbps as ½ of the given data rate; 2.4 kbps or 3.6 kbps as ¼ of the data rate; or 1.2 kbps o kbps as ⅛ of the data rate. Such a variable data rate must be detected by the receiving unit. The reverse fundamental channel generator 118 spreads the individual channels with unique orthogonal codes previously assigned thereto to discriminate the channels and spreads the signals from the users with unique PN codes to discriminate the users. Thus different orthogonal codes are assigned to a pilot channel, an access channel, a fundamental channel and a supplemental channel in order to discriminate the respective channels and the respective orthogonal codes used for every channel are shared among the users. For example, an orthogonal code used for the fundamental channel is shared among all users to discriminate the fundamental channel.
A supplemental channel generator 119 processes information received on a reverse supplemental channel and sends the received information to the base station. Information on the reverse supplemental channel includes RLP frames, packet data and the like. The supplemental channel generator 119 has a data rate of more than 9.6 kbps. The supplemental channel generator 119 has a scheduled data rate, i.e., the base station communicates with the mobile station at a data rate predetermined through negotiation with the mobile station through the dedicated control channel. The reverse supplemental channel spreads the individual channels with unique orthogonal codes previously assigned thereto to discriminate the channels and spreads the signals from the users with unique PN codes to discriminate the users. The fundamental channel and the supplemental channel will become traffic channels.
An adder 120 adds together transmission signals on the reverse link received from the dedicated control channel generator 115 and the pilot channel generator 116. An adder 121 adds together transmission signals on the reverse link received from access channel generator 117, fundamental channel generator 118 and supplemental channel generator 119. A spreading modulator 122 multiplies the transmission signals from the adders 120 and 121 by a spreading sequence and ascent frequency converts the transmission signals. A receiver 123 frequency converts the respective channel signals of the mobile station on the reverse link with a base band and then despreads the signals through multiplication of the converted signals by a spreading sequence. The construction of the reverse link channel receivers provided in the mobile station are omitted in FIG. 1.
In the CDMA communication system, as shown in FIG. 1, the base station comprises controller 101, dedicated control channel generator 102, pilot channel generator 103, sync channel generator 104, paging channel generator 107, fundamental channel generator 108 and supplemental channel generator 109. The mobile station comprises controller 114, dedicated control channel generator 115, pilot channel generator 116, access channel generator 117, fundamental channel generator 118 and supplemental channel generator 119. For the output form of the individual channel generators in the base station, the signals from dedicated control channel generator 102, fundamental channel generator 108 and supplemental channel generator 109 are two channel signals, i.e., having an in-phase channel component and quadrature-phase channel component, while only one channel signal is generated from pilot channel generator 103, sync channel generator 104 and paging channel generator 107. It is assumed herein that the only one channel component is the in-phase channel component.
Unlike the channel generators of the base station, those of the mobile station generate only one channel component. Thus the outputs of the dedicated control channel generator 115 and the pilot channel generator 116 of the mobile station are added up and fed into the spreading modulator 122 as an in-phase channel, and the outputs of the remaining channel generators 117, 118 and 119 are added up and fed into the spreading modulator 122 as a quadrature-phase channel. When using the access channel, the output of the pilot channel generator 116 is an in-phase channel input and the output of the access channel generator 117 is a quadrature-phase channel input, since the access channel generator 117 generates the output prior to generation of the traffic channel.
FIG. 3 illustrates a handoff procedure according to the prior art when the mobile station travels from a cell of the async base station to a cell of the sync base station shown in FIG. 1.
Referring to FIG. 3, in step 301, mobile station B receives from async base station A a message including information about other base stations adjacent to the async base station A through a broadcast channel or a paging channel. In step 302, the mobile station B measures the reception strengths of pilot signals transferred from the adjacent base stations and sends a message including the measurement results of the pilot signals to the async base station A through a reverse dedicated channel. Then, the async base station A analyzes the message on the reverse dedicated channel to determine whether there is a target async base station. If a target async base station exists, the async base station A confirms the handoff; otherwise, it sets parameters T, T0 and N for detecting the reception strength of the pilot signals from the adjacent sync base stations, where T0 is a time to detect the pilot signal of a sync base station, T is a time interval for detecting the pilot signal of the sync base station, and N is a parameter defining the number of times for detecting the pilot signal of the sync base station. In step 303, the mobile station B receives a direction message on a forward dedicated control channel to measure the reception strength of the pilot signals of the async and sync base stations adjacent to the async base station A, and a message including the parameters. Upon receiving the message on the forward dedicated control channel, the mobile station B measures the reception strengths of the pilot signals from the sync and async base stations adjacent to the async base station A based on the parameters T, T0 and N.
In step 306, the mobile station B detects a pilot signal received from the individual sync base stations adjacent to the async base station A. Here, the pilot signal enables the mobile station B to estimate the channels and rapidly acquire initial synchronization for new multiple paths. Besides detection of the pilot signal, the mobile station B analyzes in step 306 a sync message received from a sync base station such as sync base station C through a forward sync channel to recognize the sync base station C, and acquires system information about the sync base station C. The sync message includes system information necessary for communication with the sync base station C, such as system ID number, network ID number, PN_OFFSET value, long code information after 320 ms, and paging channel data rate. For example, the sync channel frame used in the IS-95 system is 80 ms in length with a data rate of 96 bits and comprises three sub frames having a length as long as one period of a short code. Here, the sync message including the system information about the sync base station C has a length of more than 200 bits including a message length field and CRC. Even when the message is less than 96 bits in length, the 80ms sync frame necessarily sends 96 bits by adding the surplus bits to the message. Thus the mobile station B must receive at least three 80 ms sync frames in order to receive all sync messages including the system information. Without errors in the sync messages, it takes at least 240 ms for the mobile station B to recognize the sync base station C and receive information of the sync base station C.
In step 304, the mobile station B sends a message, including the measurement results of the reception strength of the pilot signals received from the adjacent base stations and information about the sync message, to the async base station A through a reverse dedicated channel. Then, the async base station A analyzes the received message on the reverse dedicated channel and sends the measurement results to the upper network. The upper network checks the existence of the target sync base station C and sends to the async base station A a handoff direction message including information necessary for the handoff. In step 305, the mobile station B receives the handoff direction message including information about traffic channels for communication with the target sync base station C, through the forward dedicated channel from the async base station A. Once receiving the handoff direction message, the mobile station B prepares to receive traffic data from the sync base station C with reference to the traffic channel information included in the message. In step 308, the mobile station B receives null traffic or the like on a forward fundamental channel from the sync base station C to ensure stability of channels. The mobile station B receives in step 309 a traffic message on the forward fundamental channel from the sync base station C while moving to a cell of the target sync base station C, thereby switching a call service from the async base station A to the sync base station C. Thereafter, the mobile station B sends a preamble on a reverse fundamental channel to inform that transmission is successful, in step 310, and sends a handoff complete message on the reverse fundamental channel to the sync base station C, in step 311.
With the above-described forward channel structure of the conventional sync mobile communication system, the mobile station B must receive at least three sync frames on the forward sync channel of the sync mobile communication system. For example, a sync mobile communication system having the channel structure shown in FIG. 1 has a minimum reception time of 240 ms. Thus it will take at least 240 ms for the mobile station B to acquire system information for communication with the sync base station C while traveling from a cell of the async base station A to a cell of the target sync base station C. During this reception time, the mobile station interrupts communication with the async base station A. That is, taking a long time in performing the procedure of FIG. 3 results in a detrimental effect such as a loss of data communicated between the async base station and the mobile station.